In addition to the riding comfort and steering stability, improvements of the performances such as low noise characteristic and hydroplaning characteristic are demanded in a pneumatic tire. And, it is known that a tread pattern formed in the tread part is one of the factors that affect such performances.
There are such tread patterns as the lug pattern mainly composed of lateral grooves, the rib pattern mainly composed of circumferential grooves, the rib and lug pattern which intermediates the former two and the block pattern having blocks by dividing the tread part by circumferential and lateral grooves. And it is known that although, specifically, a tire having the block pattern is generally superior in characteristics against the road surface such as driving performance and hydroplaning characteristic, it is generally inferior in cornering power and wear resistance due to the relatively low rigidity of the blocks. However, in radial tires which are widely used today, as the rigidity of the treat part is increased by a belt layer having superior hoop effect, and the wear resistance and cornering power are improved by employing harder tread rubbers, tires with the block pattern are being used in high-speed buses and passenger cars.
As the applications of such tires with the block pattern are widened, however, it is required to further improve the riding comfort and steering stability of a tire without affecting the low noise characteristic and hydroplaning characteristic.
However, as the rigidity of the tread part should be generally reduced to improve the envelope performance in regard to convex run-over characteristics for the purpose of improving riding comfort, and the rigidity of the tread part should be generally increased to increase the cornering power for the purpose of improving steering stability, the riding comfort and steering stability are contradictory characteristics that could not be compatible. There has never been any suggestion for improving the riding comfort and steering stability in conventional tread patterns that could compatibly realize the two characteristics.
It is hence a primary object of the invention to provide a pneumatic tire that can improve the riding comfort and steering stability without affecting the hydroplaning and low noise characteristics, basically by mainly limiting the circumferential/lateral ratio of the blocks within a specific range.
According to one aspect of the present invention, a pneumatic tire has such tread pattern that a tread part is divided into blocks B aligned in parallel by plural vertical grooves G which extend in the circumferential direction and a number of lateral grooves g which cross the circumferential grooves G, wherein A) the block circumferential/lateral ratio Lm/Wm of 1) the average block circumferential length Lm, which is the quotient of the actual length Le in the circumferential direction, that is, the difference between the circumferential length L of the tread part and the total width Lg, Lg=Lg.sub.1 + . . . +Lg.sub.n, of the lateral grooves g in the circumferential direction divided by the umber Ng of the lateral grooves g, to the average block width Wm, which is the quotient of the actual length We in the direction of the tire's width, that is, difference between the ground-contact width W of the tire, and the total width WG, WG=WG.sub.1 + . . . +WG.sub.n of the circumferential grooves G in the direction of tire's width, divided by the added number of circumferential grooves NG+1, that is, the number NG of the circumferential grooves G added by 1, is 0.38 in a range from 0.55 to and B) the block circumferential length ratio Lm/L, which is the ratio of the average block circumferential length Lm to the circumferential length L of the tread part, is in a range from 1/120 to 1/85.
In equation format, the above relationships are as follows: EQU Lm=Le/Ng=(L-Lg)/Ng EQU Wm=We/(NG+1)=(W-WG)/(NG+1) EQU 0.38.ltoreq.Lm/Wm .ltoreq.0.55 EQU 1/120.ltoreq.Lm/L.ltoreq.1/85
In this invention, the block circumferential/lateral ratio Lm/WM is set to being a range from 0.38 from 0.55 to as mentioned hereinbefore. It has been obtained as the rate of the average block circumferential length Lm at which the envelope power EP can be reduced within a specific range so as to improve the riding comfort while controlling the pattern noise, to the average block width Wm at which the cornering power CP can be increased so as to improve the steering stability while controlling the reduction of the hydroplaning characteristic. Therefore, by specifying the block circumferential/lateral ratio Lm/Wm within the aforementioned range, the riding comfort and steering stability can be improved without affecting the pattern noise and hydroplaning characteristics.
In addition, the block circumferntial length ratio Lm/L is set in a range from 1/120 to 1/85.
The block circumferential length ratio Lm/L represents the rate of the average block circumferential length Lm that can reduce the envelope power EP so as to improve the riding comfort without affecting the pattern noise to the circumferential length L of the tread part. The average block circumferential length Lm can be practically determined by specifying the value and limiting it within the block circumferential/lateral ratio Lm/Wm .